In order to protect electronic circuit elements from surge voltage or thunder shock applied thereto, it is conventional to use a surge absorbing device such as a varistor comprising a high resistance element of metal oxide having a nonlinear characteristic of voltage or an arrester comprising a discharge gap provided between a pair of electrodes. The varistor has a high response velocity of approximately 10.sup.-9 second relative to the surge, but is compelled to have a large size in order to increase its current-proof characteristic because otherwise it has a small current-proof characteristic and also tends to self-oscillate or distort a normal signal waveform due to its large electrostatic capacity of approximately 200-800 pF. The arrester has a large current-proof characteristic and a small electrostatic capacity of approximately 2-5 pF, but has a low response velocity of approximately 10.sup.-6 second, which disadvantageously prevents the electronic circuit from being protected from steep surge.
In order to avoid the drawbacks of the prior surge absorbing devices, the inventors have previously proposed a surge absorbing device having the advantages of the prior varistor and arrester as shown in FIG. 1 (see Japanese patent application No. 30357/1983). The proposed surge absorbing device 1 comprises a substrate of high resistance element 2 and a pair of electrodes 4 and 4' having outer leads 3 and 3', provided on the periphery of the high resistance element 2 and facing each other with a discharge gap 5 formed between the electrodes 4 and 4'. The components are contained in a hermetically sealed case 7 which is filled with gaseous medium 8 for discharge. The surge absorbing device serves to absorb a surge current at high velocity in the following manner. When a surge voltage is transiently applied across the high resistance element 2, an exciting discharge occurs between the electrodes 4 and 4' due to voltage drop of the product of the resistance value of the high resistance element 2 and the value of surge current and is immediately transferred into a main discharge of large current by its excitation.
The surge absorbing device advantageously has a higher response velocity, a smaller electrostatic capacity and a smaller size relative to the prior varistor and arrester, and in addition thereto has an improved current-proof characteristic. However, in case the high resistance element as the substrate comprises metal oxide having a nonlinear or linear characteristic of voltage, it has a varied limited voltage due to variation in the resistance value of the high resistance element or in a coeffecient of nonlinearity of voltage during its manufacture or usage, making the characteristic of surge absorption unstable. This results firstly from heating, in the process of manufacture, for removal of gas out of the components, which may be accomplished by a so-called vacuum baking and which is required to stabilize the characteristic of discharge between the electrodes. It also results, secondly, from exposure of the high resistance element to the atmosphere of discharge between the electrodes, which causes the metal oxide to be reduced due to high temperature and ionic shock.